Optical disks are known as record media which a signal can be optically recorded or reproduced onto or from by irradiating a ray bundle. Generally, by irradiating a ray bundle and detecting reflected light, one-dimensional data are sequentially read out from a series of pits arranged in a direction along a track so as to reproduce a recorded signal.
With two-dimensional data having information in two longitudinal and transverse directions, the amount of information to be recorded is dramatically increased, and hence, for example, two-dimensional code is widely used for an image and read in by a two-dimensional image sensor to reproduce recorded data.
Meanwhile, holograms are attracting attention as record media capable of recording two-dimensional data at high densities. The characteristic of the holograms consists in recording wave fronts of light carrying information volumetrically as change in the index of refraction (a hologram) in a recording medium made of a photosensitive material. Performing multiplex recording on a hologram recording medium can dramatically increase the recording capacity. For example, as shown in FIG. 27, in an angle multiplexing scheme, by changing the relative angle between reference light and signal light with respect to a hologram recording medium, a plurality of holograms (e.g., holograms of reference light of incident angles A, B and signal light) can be recorded in the same area of the hologram recording medium 2.
A hologram apparatus of recording and reproducing into and from hologram recording media comprises, for example, a laser source to generate coherent light; a spatial modulator to spatially modulate the coherent light according to two-dimensional data, thereby generating signal light; an optical system to irradiate the coherent reference light and the coherent signal light onto a recording medium through an object lens, thereby forming a hologram inside the recording medium; and an optical system to irradiate reference light, which has an image sensor to receive and detect an image of two-dimensional data imaged by the reference light through an object lens. The image sensor is a CCD (charge-coupled device) or a photoelectric conversion device constituted by an array of CMOSs (complementary metal-oxide semiconductor devices) or the like.
When reproducing record data from recorded two-dimensional data, the quality of a read-in image, i.e., two-dimensional data is important. Accordingly, a positioning mark is contained in two-dimensional data and is displayed in the same shape and at the same place with its shape and location being fixed. For example, the same specific symbol is displayed in one or more corners or the like of the rectangular two-dimensional data.
In the hologram apparatus, the positional deviation between the center of the aperture area of the object lens receiving light and two-dimensional data recorded in a hologram recording medium is estimated based on the positioning mark in an image of the two-dimensional data read in from the hologram recording medium, and their positions are corrected such that the center of the two-dimensional data coincides with the center of the aperture area of the object lens. One example of the patterns of a reference position mark (positioning mark) is a plurality of crosses arranged on the edge of a rectangle to which the aperture area of the object lens is inscribed (refer to Japanese Patent Application Laid-Open Publication No. 2005-227704, paragraph 0055).
Because the positioning mark in the two-dimensional data is displayed at the same location and in the same shape, and because the light output of the light source of the hologram apparatus when recording into a hologram recording medium is considerably large while it depends on the sensitivity of the hologram recording medium, a burning phenomenon of the positioning mark occurs in optical components such as the object lens (including particularly the image sensor at the imaging position). This burning phenomenon occurs because the surface coats or members themselves of optical components of the apparatus absorb light energy, and the transmittance or the like thereof changes from their normal state. If the burning occurs, an image of the positioning mark reproduced and imaged on the image sensor and an image of the positioning mark due to the burning exist mixed. As a result, it becomes difficult to read in the positioning mark reproduced and thus to read reproduced data.
Patent Document 1:
Japanese Patent Application Laid-Open Publication No. 2005-227704